1. Field of the Invention
The present invention relates to voltage controlled oscillators (VCOs), particularly wideband VCOs.
2. State of the Art
FIG. 1 shows typical complementary LC tuned VCO 100 with a single tuning port configured for wideband operation. A complementary cross coupled CMOS inverter is formed by PMOS transistors M1, M2 and NMOS transistors M3, M4. The PMOS transistors are cross coupled such that a gate electrode of each is coupled to a drain electrode of the opposite PMOS transistor. Similarly, the NMOS transistors are cross coupled such that a gate electrode of each is connected to a drain electrode of the opposite NMOS transistor. The PMOS transistor M1 and the NMOS transistor M3 are coupled drain to drain, and the PMOS transistor M2 and the NMOS transistor M4 are coupled drain to drain. Source electrodes of the PMOS transistors are connected to a supply voltage VDD. Source electrodes of the NMOS transistors are connected through a tail current source I to ground. Complementary output signals are formed between the PMOS transistor M2 and the NMOS transistor M4, on the one hand (output terminal C), and between the PMOS transistor M1 and the NMOS transistor M3, on the other hand (output terminal CZ). Coupled between the output terminals are frequency controlling reactive elements including an inductor L, a capacitor C and varactors V and Vz. A tuning input signal (TUNING) is connected to control terminals of the varactors at node X.
Wide band tuning implies high tuning gain which is undesirable for noise considerations, as may be appreciated from the following example. For a VCO to cover the range from 800 MHz to 1700 MHz, at 3V operation, the tuning gain will be 300 MHz/V. In this instance, 1 mv of noise on the tuning node will translate to 300 kHz of frequency deviation (phase noise).
FIG. 2 shows a typical complementary LC tuned VCO with a digital, “coarse” tuning port which divides the band into a series of sub-bands, and an analog, “fine” tuning port which functions as the tuning port for a PLL. In this case, since the PLL only operates in a sub-band, the tuning gain is reduced to the sub-band width divided by the supply.
FIG. 3 shows the frequency breakdown of the prior art VCO of FIG. 2 for the case of eight sub-bands. Note that:
1. Each coarse selected sub-band must overlap, with fixed varactor. Since all frequencies of the total band must be achievable, there can be no gaps from one selected band to the next, implying each sub-band must have overlap.
2. The amount of overlap depends on the number of sub-bands and on process/temperature/voltage variation. If a selected frequency is near the top or bottom of any sub-band, the sub-band overlap must be sufficient such that the tuning voltage can maintain the desired frequency.
3. Operation in sub-band overlap typically causes charge pump to operate outside optimum output tuning voltage for lowest spurs (i.e., near either rail) due to finite output resistance of the devices.
4. Each coarse tuned sub-band progressively compresses as frequency decreases, with fixed varactor. FIG. 3 shows the band width for each sub-band using a fixed varactor. The right hand scale shows the tuning gain plotted at each sub-band assuming 3V operation. Note that the gain varies by almost an order of magnitude.
5. Sub-band compression forces higher frequency sub-bands to have higher tuning gain, since a minimum gain must be used when designing a PLL loop. As a consequence, there will be only one sub-band with optimum gain.
6. Tuning gain is proportionally dependent on center frequency. That is, as the desired frequency increases, the tuning gain increases at approximately the same rate, as shown in FIG. 3.
Although not illustrated in FIG. 3, the VCO of FIG. 2 experiences amplitude loss across the range. VCO amplitude decreases at lower frequency sub-bands due to increased capacitive loading of the fixed drive amplifier. The amplitude is inversely proportional to load, and proportional to frequency. Automatic amplitude control (AAC) can be used at the cost of increased die area and increased noise, especially within the loop bandwidth of the control circuit. An alternative to AAC is to program the current source when programming frequency. In both the AAC case and the programming case, the use of a current source contributes significant noise to the VCO through channel noise of the source, and the 2x fundamental located at the common node. In order to keep channel noise low, the current source transistors are typically very large, adding to die area.